I agree that you shouldn't give it back. If they really needed the money, they would have kept it. Obviously, it meant more to them to give you something (regardless of how "insignificant" it might have been) which speaks volumes about how much you mean to them. Accept it gracefully. If you return it, you'll look ungrateful and inconsiderate of the idea that they wanted to give you something. Think about it this way: If you had very little, spent what you could to give them something and they returned it. How would you feel? Regardless of the circumstance (in this case, trying to justify it as being kind) it still seems insincere. If you really want to feel better about it, use the money for something that has meaning or can be useful. Don't spend it on something superficial or flashy. Use it for something that will be beneficial for your well being so that those who gave it to you can feel good about trying to help.

I don't agree that the picture is a good idea though. A simple note expressing your appreciation should suffice. Anything too "fancy" will seem artificial.